As seen in the industry, hearing aids are being designed according to smaller dimensions. These hearing aids include Behind-The-Ear (BTE), In-The-Ear (ITE), and Receiver-In-Canal (RIC). Newer types of hearing aids (RIC's) place the receiver in the ear canal. In general, an in-ear device, such as those mentioned above, is limited by certain constraints, such as, for example, comfort. Comfort may be achieved in two ways. In a first method, a shell may be custom molded to the contours of the individual ear canal. This shell houses the in-ear acoustic device. In this case, space available for the in-ear device is constrained by the requirement that the device must not protrude from the shell on any of the surfaces that are contoured to match the canal shape.
In a second method, the in-ear device may be partially encompassed by a compliant material called an ear-dome or tulip. The ear dome prevents the hard material of the in-ear device from contacting the canal walls and serves to align the in-ear device along the axis of the ear canal. The ear dome may have features to allow air to communicate between the tympanic membrane (TM) and environment around the user's head for a vented or open-ear response, or may provide an effective seal to air flow. In the sealed configuration, the acoustic outlet must be on the TM side of the seal.
The ear canal is typically not a straight conduit; it may have bends in it. In addition, the cross sectional shape and area vary with distance toward the TM. These features are unique to each individual and ear. It is a challenge to comfortably fit a hard object of some nominal length and effective diameter into individual ear canals. Moreover, it is desirable from a manufacturing and distribution standpoint to have an in-ear device design fit comfortably into the largest percentage of potential wearers as possible (referred to as the “fit rate”). In general, the fit rate of an in-ear device decreases with increasing device length.
The sound pressure generated by a receiver operating directly into the ear canal (that is, where the acoustic channel is nonexistent or provided only by the formed metal tube typically attached to the port of receivers) has at least one peak at the mechanical resonance frequency of the receiver, generally around 3 kHz. A second resonance may occur at or above 10 kHz caused by the effective inertance of the air in the port (and residual acoustic channel of the metal tube, if present) resonating with the effective compliance of the front volume. A deep valley exists between the two response peaks exhibited by these resonances. It is often desirable to have a lower peak-to-valley ratio. The peak-to-valley ratio can be reduced by introducing an acoustic channel between the port and acoustic outlet. The acoustic channel is an acoustic transmission line between the port and acoustic outlet. In a simple analysis, this acoustic transmission line can be represented by a simple inertance (mass), which allows for shifting the frequency of the acoustic resonance by adding inertance to the system, by means of an acoustic channel
The acoustic channel creates an additional acoustic load upon the receiver, thereby modifying its output. These two points of view (channel modifies receiver through loading, or channel modifies acoustic output through the transmission line) are consistent with and mathematically equivalent to each other.
The acoustic channel (viewed as a transmission line) will introduce a time delay between the acoustic outlet and the port, equal to the effective length of the acoustic channel divided by the speed of sound. This provides a definition of the effective length of the acoustic channel. An acoustic channel with a relatively small cross-sectional dimension that is much larger than a wavelength can be considered lossless, meaning that the sound will not attenuate as the wave propagates down its length. However, at smaller dimensions, the acoustic wave begins to exchange heat with the walls of the acoustic channel, thereby attenuating the wave. This is exhibited in the frequency response as reduced amplitude of the acoustic peaks and is identified as damping.
To a reasonable degree of accuracy, the behavior of the acoustic channel can be represented by a lossy transmission line parameterized by its cross-sectional area and length. Thus, area and length of the channel are independently important in the design of the acoustic channel. An acoustic channel with area that varies with length can be segmented and represented by a series of transmission lines; other analysis methods also exist. By varying the area along the length of the channel, the acoustic channel may also be designed to act at least partially as an acoustic impedance matching element between the port and the acoustic impedance presented at the outlet.
In the current state of the art, an acoustic channel is provided by attaching a length of tubing to the port of a receiver. The other end of the tubing functions as, and is referred to, as the acoustic outlet. In a Behind The Ear device (BTE), the receiver is attached to a tube (typically flexible for feedback control reasons), which is attached to an earhook assembly, and having a channel formed in its interior. The earhook assembly then is attached to a clear, flexible tube through a custom-molded earmold. This provides a relatively long acoustic channel, causing many acoustic resonances. Tube segment areas and lengths can be chosen to provide a wide bandwidth response and peak-to-valley ratios well within the range of acceptability.
In an ITE device, a short, flexible tube is attached to the port of the receiver and to a canal end of the ITE. This tube is approximately 1 mm to 2 mm in diameter and somewhere between 3 mm and 10 mm in length. The actual length is usually chosen during an assembly phase of the hearing aid to place the receiver within the shell in such a way that the receiver case does not contact the shell.
In a RIC device, a secondary body of the hearing instrument separate from the main body houses the receiver or receiver motor. A short, cylindrical length of the housing is allowed to protrude in front of the receiver to act as a channel. Typically, this is between 1 mm and 3 mm in diameter and about 2 to 3 mm in length. This protrusion is also the feature over which the ear dome section of the hearing instrument fits, and may have ridged features to help prevent the ear dome from accidently slipping away. In particular, this style of acoustic channel is ineffective at modifying acoustic resonances. The channel is too short to have any useful effect, and increasing its length is quite difficult due to fit rate considerations. In all cases, wax protection devices or acoustic dampers may be added at the acoustic outlet or along the length of the channel.
A need, therefore, exists for modifying the frequency response of an in-ear device with a minimal increase in in-ear device length, thereby maintaining an acceptable fit rate.